The present invention relates to the deicing of the air inlet cowls of jet engines, particularly aeroengines.
It is known that, if need be (to prevent ice from forming or to eliminate ice which has already formed), the leading edge of the air inlet cowl of such engines is deiced by heating it using pressurized hot air tapped off said engine and conveyed to said leading edge by a hot air circulation circuit. This pressurized hot air, tapped off the engine, is at a high temperature, for example of the order of 400xc2x0 C., which means that said pipe radiates heat and that the structures surrounding said air inlet cowl which are sensitive to heat (for example the acoustic panels made of composite) need to be protected against the heat. What is more, for obvious safety reasons, it is also necessary to protect said surrounding structures against the event of leaking pressurized hot air or bursting of said pipe.
To this end, document U.S. Pat. No. 6,241,189 discloses an air inlet cowl for a jet engine, particularly an aeroengine, said air inlet cowl being provided with means for deicing its leading edge, and comprising for this purpose:
a hollow leading edge delimiting an internal peripheral chamber, closed by a first internal partition, and provided with at least one exhaust orifice placing said internal chamber in communication with the outside;
a hot air feedpipe capable of being connected, at its rear end away from said leading edge and passing through a second internal partition, to a pressurized hot air circuit and, at its front end toward said leading edge, to an injector injecting said pressurized hot air into said internal chamber;
a protective internal casing collaborating with said first and second internal partitions to delimit an isolation volume enclosing said feedpipe;
at least one opening for introducing air into said volume; and
at least one opening for extracting air from said volume.
Thus, in this known air inlet cowl, said pipe, with its connections and flanges, is isolated from the remainder of the inside of the air inlet cowl and said (continuous and integral) protective internal casing affords protection to the surrounding structures against thermal radiation and pressurized hot air leaks, and against the effects of said pipe bursting. By virtue of the openings for introducing and extracting air there is obtained, in normal operation, permanent internal ventilation of the isolation volume, and this limits the radiation of heat from the feedpipe, the heat-sensitive surrounding structures thus finding themselves protected from any damage or ageing associated with exposure to high temperatures. In the event of leaks, bursting or explosion of the pipe, the hot air is evacuated to the outside through the extraction opening, so that, here again, said surrounding structures are protected against the pressurized hot air.
This known air inlet cowl therefore perfectly performs its functions of protecting said surrounding structures from heat. However, it must be observed that, in the event of damage to the feedpipe, the pressurized hot air is immediately and in its entirety sent to the outside via said extraction opening, the cross section of which is, incidentally, calibrated to avoid the overpressure resulting from the bursting of said pipe being detrimental to the protective internal casing. Thus, this hot air is not only wasted to no effect and, on escaping, generates an increase in drag, but also ceases to be fed into the injector and the internal peripheral chamber, which means that deicing of the leading edge of said cowl can no longer be provided. It is therefore no longer possible for the pilot to take steps to prevent the formation of ice or to eliminate a layer of ice which has already formed, with all the risks that that entails, such as damage to the engine by ingestion of pieces of ice, for example.
It may readily be observed that the air inlet cowl described in document U.S. Pat. No. 4,757,963 has the same drawbacks.
It is the object of the present invention to overcome these drawbacks and to provide satisfactory deicing of said leading edge of the air inlet cowl, even in the event of serious damage to said hot air feedpipe.
To this end, according to the invention, the air inlet cowl for a jet engine, particularly for an aeroengine, said air inlet cowl being provided with means for deicing its leading edge, comprises:
a hollow leading edge delimiting an internal peripheral chamber, closed by a first internal partition;
at least one exhaust orifice placing said internal chamber in communication with the outside and made in said hollow leading edge;
a pressurized hot air feedpipe capable of being connected, at its rear end away from said leading edge and passing through a second internal partition, to a pressurized hot air circuit and, at its front end toward said leading edge, to an injector injecting said pressurized hot air into said internal chamber of the leading edge;
a protective internal casing collaborating with said first and second internal partitions to delimit an isolation volume enclosing said feedpipe; and
an air extraction opening made in said first partition to place said isolation volume enclosing said feedpipe in communication with said internal peripheral chamber of said leading edge, the cross-sectional area of this extraction opening being chosen from a range of values such that, in the event of damage to said hot air feedpipe:
the elimination of pressurized hot air in said isolation volume is not detrimental to said protective internal casing; and
the injector injects into said internal chamber a pressurized hot air flow rate at least equal to 50% of the hot air flow rate in said feedpipe.
Thus, in the event of damage to said feedpipe, the pressurized hot air spreads out in said isolation volume and passes into said peripheral chamber of the leading edge through said air extraction opening. Because of the special calibration, according to the present invention, of said air extraction opening, on the one hand, the overpressure resulting from the bursting of said pipe cannot damage the walls of said isolation volume and, on the other hand, the pressure therein increases to become at most equal to the hot air pressure inside said feedpipe. The result of this is that, at the zone where the feedpipe is leaking, the pressure in said isolation volume acts as an imaginary bung virtually plugging said leaking zone and confining the pressurized hot air, tapped off the engine, in the feedpipe. Said hot air therefore continues, at least in part, to feed the injector, which allows satisfactory deicing of the leading edge of the air inlet cowl. The hot air which has circulated through said peripheral chamber of the leading edge and has cooled down on contact with the walls thereof is then removed to the open air, without excessive pressure drop, to said exhaust orifice with which said leading edge is provided.
In addition, the stream of hot air passing through the extraction opening mixes with the stream of hot air injected by the injector and is carried along by the latter, which allows the former to play a part in deicing the leading edge of the air inlet cowl.
Of course, in normal operation, when said hot air feedpipe is neither burst nor leaking said extraction opening made in the first partition exhibits no disadvantage, it merely placing two hot enclosed areas, namely the isolation volume and the peripheral chamber of the leading edge, in communication.
It will be noted that document U.S. Pat. No. 5,400,984 describes an air inlet cowl for an aircraft jet engine which, on the face of it, seems similar to that of the present invention. It will, however, be noted that, in the latter document:
said air extraction opening allows free, wholly unimpeded, gaseous communication, between said isolation volume and the hollow leading edge; and
in addition, the orifice placing the internal chamber of the hollow leading edge in communication with the outside is made not in the leading edge but rather in the skin structure of the rear compartment, an elbowed pipe providing the communication between said internal chamber and said orifice.
Now, it is delusional to think that an extraction opening such as the one provided by U.S. Pat. No. 5,400,984, can be used to replace the injector in the event of said feedpipe bursting. What actually happens in that case is that the pressurized hot air from the burst feedpipe and filling said isolation volume immediately expands as it passes into said peripheral chamber of the leading edge through said extraction opening and loses its kinetic energy. All it can then do is to spread out in a zone close to said extraction opening, without being able to circulate throughout the internal peripheral chamber of the leading edge. The result of this then is that the zones of this chamber which are distant from the extraction opening remain subjected to relatively low temperatures, whereas those situated in the vicinity of said extraction opening are raised to very high temperatures. It is therefore impossible, under these conditions, to obtain satisfactory deicing of the leading edge. This detrimental effect is further amplified by the significant pressure drop introduced by said elbowed pipe.
In contrast, in the leading edge cowl according to the present invention, the exhaust orifice is made directly in the hollow leading edge, which reduces to a minimum the pressure drop of the stream of hot air exhausted to the outside and, in the event of damage, the injector continues to be useable, in spite of the bursting of the feedpipe. As a result, injected to the peripheral chamber of the leading edge is a stream of pressurized hot air which starts to circulate in this chamber and, at every moment, the stream of pressurized hot air being injected mixes with the stream of air already circulating therein. Thus, by virtue of the present invention, the temperature of the air stream in said peripheral chamber can be evened out and satisfactory deicing can be obtained, even in the part of the cowl away from said injector.
To further improve the effect of the stream of hot air passing through said extraction opening being carried along by the stream of hot air injected by the injector, it is advantageous to provide, inside said internal peripheral chamber of the leading edge, a deflector capable of deflecting the hot air passing through said extraction opening in the same direction as the hot air injected by said injector.
Thus, the hot air which has passed through said opening does not expand immediately after it has passed therethrough, but, on the other hand, takes advantage of the presence of said deflector to also circulate and mix with the hot air already in motion in said internal peripheral chamber. It may be noted that the dimensions of said deflector may remain small enough by comparison with the dimensions of said chamber that the presence of this deflector does not disrupt the flow of deicing air already present in said chamber.
In the air inlet cowl according to the present invention, said air extraction opening may be formed of a single orifice. However, in order to accelerate, within the internal peripheral chamber of the leading edge, the evening-out of the temperature between the pressurized hot air stream being injected into the chamber and the stream of air already circulating therein, and in order not to damage the surrounding structures, it may be advantageous for the extraction opening to be formed of a number of orifices, the sum of the passage cross-sectional areas of which corresponds to the passage cross-sectional area of said single orifice. Thus, firstly, as the stream of hot air passing into the peripheral chamber of the leading edge through the number of air extraction orifices is no longer concentrated at a single point, there is no risk of the zone close to said orifices being damaged by the high temperature of said stream. Secondly, the various jets of air resulting from the presence of the number of extraction orifices mix more quickly with the stream of air already circulating in the chamber and therefore without danger to the surrounding structures.
Moreover, said air extraction opening may either be away from the pressurized hot air feedpipe or may surround the latter. In the latter instance, it may then be in the form of a single annular orifice or of a number of orifices distributed around said pipe.
As described in document U.S. Pat. No. 6,241,189, said protective internal casing may collaborate, in order to delimit said isolation volume enclosing said feedpipe, not only with said first and second internal partitions, but also with the internal face of the external wall of said air inlet cowl. However, because said protective internal casing may have good thermal insulation properties, and be effective enough to thermally protect said surrounding structures, it is possible to reduce the bulk of the protective internal casing, and therefore its mass, by reducing said isolation volume. To do this, the protective internal casing may have a tubular shape delimiting, with said feedpipe, an isolation volume of annular cross section.
Thus, the present invention makes it possible to provide solutions to all the problems encountered in engine air inlet cowls, concerning:
systematic deicing of the leading edge of said cowls even in the event of air leaks from the feedpipe;
resistance to high temperatures; and
ease of repair and inspection; specifically, by making the fitting of said casing inside said air inlet cowl removable, the parts of which the heat protection is made can be inspected, as can the surrounding parts (structure and systems).
The figures of the appended drawing will make it easy to understand how the invention may be achieved. In these figures, identical references denote similar elements.